A motorized drive system may drive motion of an output element over a defined path range (stroke). For example, a motor-driven actuator is commonly used to move an aircraft flight control surface (e.g. a flap or a slat) or an aircraft door in a bidirectional manner between an extended position and a retracted position. The control system commanding the drive motor imposes travel limits on the rotary drive system. In addition, a mechanical stop module is commonly provided as a safeguard to prevent overtravel in the event the motor control system experiences a failure. The stop module functions to stop rotation of the rotary drive system when the rotary drive system reaches an end-of-stroke position in one travel direction, and allows counter-rotation of the drive system motor for travel in the opposite direction.
One known stop mechanism includes an end stop provided on an arm of an output bell crank actuated by a rotary drive system, and a detent surface arranged to engage the end stop to limit the angular stroke of the bell crank. Because the stopping torque is applied to the bell crank arm, a torque limiter is needed to protect the system from very high torque levels. Consequently, this type of mechanism adds weight and complexity to the rotary drive system.
Other known mechanisms for preventing overtravel use a travelling nut assembly limited by end stops. For example, U.S. Pat. No. 4,064,981 discloses a stop mechanism having a travelling nut assembly that contacts a shock absorbing stop to terminate shaft rotation by frictionally jamming screw threads between the nut assembly and the driven shaft. Another known stop mechanism includes a moving nut having “dog stops” on an end face of the nut that are engaged by mating dog stops on an end stop to prevent further rotation when the nut reaches an end-of-stroke position. Mechanisms that rely on a travelling nut are generally proportional in size and weight to the number of motor rotations per stroke. Consequently, travelling nut systems may be too large and/or heavy in some applications, particularly aircraft applications. A differential drive may be arranged to help reduce screw thread length in a travelling nut system, however this increases the complexity of the system, making it more expensive to design, make, and assemble.
U.S. Pat. No. 4,867,295 discloses a travelling nut stop mechanism that includes a separate screw shaft along which the nut travels, wherein the screw shaft is driven by the rotatable drive shaft. A rotating stop and a stationery stop are provided at one end of the screw shaft, and a bearing housing is provided at the other end of the screw shaft to support the screw shaft for rotation and axial movement. The travelling nut is moveable in one direction to engage the rotating stop and stationery stop, thereby stopping rotary motion of the screw shaft. The travelling nut is moveable in the other direction into contact with the bearing housing to exert an axial force upon the screw shaft such that the screw shaft moves axially to engage the rotating and stationery stops, whereupon rotary motion of the screw shaft is terminated. The screw shaft acts as a torsion shock absorber upon the termination of the rotary motion of the screw shaft by the rotating and stationery stops. Much like the mechanisms mentioned above, the weight, size, and complexity of the system are limiting factors.
Other known of stop mechanisms use a Geneva drive to count motor rotations and activate a pawl to pop into engagement or apply dog stops at the end-of-stroke. Examples of this type of stop mechanism are found in U.S. Pat. Nos. 4,641,737 and 4,721,196. Such systems are mechanically complex, and torsional shock absorbing means may be needed. If this type of stop mechanism is used at or near the motor in a system where there are many motor rotations between end-of-stroke limits, the counting mechanism becomes large, heavy, and/or complicated.
Another known approach for preventing overtravel relies on valving or switches to halt power to the drive motor to prevent motion beyond an end-of-stroke limit. However, this approach does not prevent manual driving from over running normal end-of-stroke limits.